This invention relates to forming carbon fibers. More particularly, this invention relates to an apparatus for making carbon fibers in a continuous gas phase reaction, which provides desired conditions for each step of the reaction including decomposition of a compound containing nuclei, formation of nuclei, decomposition of a carbonaceous compound and reaction of the decomposed carbonaceous compound with the nuclei, thereby forming carbon fibers.
Carbon fibers are used to make composite materials. Such carbon composites provide the advantages of relatively high strength and low weight as compared to other materials commonly used to form articles such as automotive, boat, airplane and other parts. High strength and low weight are key advantages needed to reduce vehicle fuel consumption and increase fuel economy. The strength of the composite is directly related to the quality of the carbon fibers. Preferred fibers have basal planes arranged in concentric circles and are of microscopic size for improved strength to weight ratio. Such microscopic fibers effectively achieve the advantages of improved strength, low weight and, therefore, improved fuel economy.
Prior methods of forming fibers basically consist of conducting a gas phase batch reaction in a vessel. Specifically, such methods include dispensing iron particles, in the form of a powder or in a solution, into a vessel while simultaneously dispensing a liquid or gas hydrocarbon compound, then decomposing the hydrocarbon compound to form carbon-containing compounds which react with the iron particles. A diluent gas, such as hydrogen, is injected into the vessel to control the rate at which decomposition and growth proceed. The vessel is typically arranged so that convective currents are generated. These currents often cause a loss of heat which reduces the temperature of the reactants to below that desired for optimum fiber formation. The currents also cause losses of feedstock reactant material, including expensive catalyst material, due to thermophoretic deposition on the walls. Typically, the batch reaction has proceeded in the vessel for a period of time, fibers are collected, and then the reaction is started again. Typically, large fibers, that is fibers having dimensions greater than the desired microscopic fibers, are produced along with undesirable soot. Often a mixture of soot and large fibers results when prior processes and vessels are used. This may occur because it is difficult to control the conditions at which the various steps of the reaction occur in conventional batch processes, particularly when convective currents arise in conventional vessels.
Prior methods and vessels typically include a preconditioning period. The vessel is preheated and/or the surface thereof is prepared using iron particles before the reaction is initiated. The prepreparation effort is typically extensive in relation to the reaction period during which the product is actually produced. Such methods are not cost effective for use in commercial scale plants.
Therefore, it is an object of this invention to provide a continuous process and apparatus for forming carbon fibers in a gas phase reaction; to improve the process of forming carbon fibers by providing an apparatus constructed and arranged to achieve optimum conditions for forming microscopic carbon fibers in a gas phase reaction; to maintain essentially steady state, controlled reaction conditions in a continuous flow process in which microscopic carbon fibers are formed; to minimize convective currents; and to minimize thermophoretic losses of feedstock reactant material. It is also an object to provide a method and apparatus which produces an increased yield of carbon fibers, provides a larger proportion of smaller fibers, substantially eliminates soot, and decreases the cost of producing carbon fibers.